Adding flame retardant material in the form of fine particles to plastics and dispersing those fine particles through the whole plastic article or in its surface regions is known. As an example, German patent specification DE-A-101 45 093 describes polyphosphates of organic nitrogen bases such as guanidine and melamine or melamine derivatives as a flame protection agent (retardant) in plastics and as a flame proofing agent in textiles. International patent WO-A-00/02869 describes polyphosphate salts, in particular in glass fibre-reinforced polyamides and polyesters, as flame protection agents. Other organic nitrogen bases are proposed in many other publications as flame protection agents, the tendency in this branch of the industry being to use halogen-free flame protection agents such as aluminium hydroxide, magnesium hydroxide or melamine cyanurate for that purpose.
When such flame protection agents are distributed in halogen-free thermoplastic plastics, a disadvantageous effect may occur which changes the properties of the plastic in an undesirable manner.
In many fields of application, it is desirable to make plastic articles which are flame-proof and capable of being written or marked upon using laser light. There is a practical problem, however, as halogen-free flame-proof flame protection agents do not exhibit sufficient contrast, and so they turn out to be of no practical use.